News release

From: Associate Professor Monica Gerth (author of the research)

Less digging, better detection: a new way to sample for kauri dieback

New research shows kauri dieback can be detected with much less digging, and reveals a pattern in the soil that could help protect at-risk trees sooner.

WELLINGTON, New Zealand — Kauri dieback is one of the most serious threats facing Aotearoa New Zealand's kauri forests. The disease is caused by a soil-dwelling microbe, Phytophthora agathidicida, which infects kauri roots and eventually kills the tree. There is no cure, so finding infected trees early and managing them well is critical to protecting the kauri that remain.

The trouble is, sampling for kauri dieback has always meant digging (a lot of digging). Standard protocols call for up to a kilogram of soil, dug from holes as deep as 20 centimetres, right where kauri's shallow, delicate roots live. A research team in the School of Biological Sciences at Victoria University of Wellington, led by Associate Professor Monica Gerth with PhD student Jade Palmer and researcher Emily Hocking, wanted to know whether sampling could be done with a much lighter touch, while also improving detection.

To find out where the microbe was most concentrated in the soil, the team collected soil cores from around kauri trees in Northland forests. Using a DNA-based test they published last year, the team found the microbe's DNA showed up consistently in the top 10 centimetres of soil. That finding held up when the team then tested this in the field with the Kauri Ora team of Te Roroa iwi, whose rohe (tribal area) includes Waipoua Forest — home to Tāne Mahuta, the largest known living kauri. Comparing shallow and deep soil samples side by side, the shallow samples consistently picked up more of the microbe than the deep ones did.

"We can reliably detect the microbe from a shallow soil sample of just 10–15 grams, taken from the top 10 centimetres," said Associate Professor Gerth. "That's a fraction of the soil, and the digging, that current methods need. It's better for the tree, easier for the people doing the sampling, and, in our data, it actually found the microbe more reliably too."

An unexpected clue for managing the disease

While mapping where the microbe sits in the soil, the team noticed a pattern. Even in trees that looked completely healthy, the microbe was sometimes already present in the soil at low levels — a possible early-warning sign. In trees with non-specific symptoms, moderate levels of the microbe helped point to kauri dieback specifically as the culprit. And around badly diseased trees, levels spiked dramatically, up to 150 times higher than around trees that looked completely healthy. That suggests these trees are shedding large amounts of the microbe into the surrounding soil, putting neighbouring kauri at risk.

"What's exciting is that this isn't just a yes-or-no test anymore — the amount of microbe we detect tells us something,” said Associate Professor Gerth. "It gives us a way to prioritise: put the most effort into the trees that need it most, whether that's a tree showing early warning signs worth watching, or one badly affected enough to fence off before it infects its neighbours. That's the kind of evidence agencies, councils, and mana whenua need."

Ian Mitchell (Te Uri Taniwha, Te Hikutu, and other hapū of Ngāpuhi), a long-time collaborator of the research team, described the results as "a big step forward for kauri health."

"The approach aligns with our tikanga: we tread softly in the presence of our forest ancestors, 'tickling their toes' in the sampling process rather than chopping them off. Having seen its potential, we are already applying these methods in our rohe to guide our protection efforts," he said.

The research was supported by Tiakina Kauri, part of Biosecurity New Zealand, and is published in the journal Microbiology Spectrum.